DNA Polymerase Beta Participates in Mitochondrial DNA Repair

We have detected DNA polymerase beta (Pol beta), known as a key nuclear base excision repair (BER) protein, in mitochondrial protein extracts derived from mammalian tissue and cells. Manipulation of the N-terminal sequence affected the amount of Pol beta in the mitochondria. Using Pol beta fragments, mitochondrion-specific protein partners were identified, with the interactors functioning mainly in DNA maintenance and mitochondrial import. Of particular interest was the identification of the proteins TWINKLE, SSBP1, and TFAM, all of which are mitochondrion-specific DNA effectors and are known to function in the nucleoid. Pol beta directly interacted functionally with the mitochondrial helicase TWINKLE. Human kidney cells with Pol beta knockout (KO) had higher endogenous mitochondrial DNA (mtDNA) damage. Mitochondrial extracts derived from heterozygous Pol beta mouse tissue and KO cells had lower nucleotide incorporation activity. Mouse-derived Pol beta null fibroblasts had severely affected metabolic parameters. Indeed, gene knockout of Pol beta caused mitochondrial dysfunction, including reduced membrane potential and mitochondrial content. We show that Pol beta is a mitochondrial polymerase involved in mtDNA maintenance and is required for mitochondrial homeostasis.